1. Field of the Invention
The present invention relates to image determining methods and apparatus which determine the type of the original image on the basis of the image signal read by a facsimile device or the like, and more particularly to methods and apparatus which discriminate clearly between a character/line picture and a dot matrix photograph image and performs efficient image processing, whether both the images are present mixed in an image.
2. Description of the Prior Art
Many halftone images and color images present around us are often prints using a dot matrix photograph, which is used to express the density of an original document in the field of printing and causes human eyes to feel the density of a print depending on the size of ink dots or the number of ink dots per unit area. The period of ink dots varies from a very short one to a very long one.
Generally, a halftone image or a color image contains a regular continuous density photograph image while in a general image processing technique, it is determined whether the image is a dot matrix photograph or a continuous density photograph depending on whether an edge as the pixel is detected. Such image determination is realized relatively easily.
If a dot matrix photograph image is scanned for reading through an image reader such as a facsimile device and the original image is tried to be reproduced by using dither processing, a low frequency beat or moire pattern will appear in the reproduced image to thereby render the image very unclear by interference of a particular frequency of the dot matrix of the read original image and the repetition frequency component of the dither matrix since the dot matrix image itself includes a dot image having periodicity.
Usually, in order to cope with the appearance of such moire pattern, a method is employed in which a change in the gradation of the dot matrix image is converted to a characteristic similar to that of a regular continuous density photograph image, and the resulting signal is subjected to dither processing to reproduce an image. According to such processing, frequency interference would not occur nor would a moire pattern appear in the reproduced image. Such image processing itself can be performed also in the regular continuous density photograph image.
Other images discriminated from a continuous density photograph image in accordance with the detection of the image edge are so-called characters/line pictures including characters/patterns. These character/line pictures are different from the dot matrix images. When the characters/line pictures are reproduced as images, their edges must be emphasized appropriately and binarized as a signal indicative of one of white and black pixels. Thus, the character/line pictures are required to be subjected to image processing different by themselves from the dot matrix image.
Prints handled usually by facsimile devices can include only character/line pictures or only dot-matrix images, but generally, include mixed character/line pictures and dot matrix images in many cases. When an image which includes mixed character/line pictures and dot matrix images is reproduced, undesinably, the filtering preferable for the dot matrix image would deteriorate the image quality of the character/line picture whereas edge emphasis preferable for the character/line pictures would deteriorate the image quality of the dot matrix image.
Thus, in order to reproduce an image of mixed character/line pictures and dot matrix images with high quality, it is desirable that these character/line pictures and dot matrix images are discriminated beforehand from each other and image processing appropriate for the respective image types should be performed in accordance with the discriminations, but there are no conventional methods and apparatus for preferable for such image discrimination.
There is a method of discriminating a dot matrix image area from an image signal using high frequency components of the image signal extracted by Fourier transform, but this method has a difficulty in practicality for the following reasons:
(a) Processing is complicated;
(b) A memory of enormous capacity is required; and
(c) An area to be referred to is large.